1. Field of the Invention
The present invention relates to etching of titanium or tungsten alloys, especially in microelectronics fabrication of N structures.
2. Description of the Related Art
C4 is an advanced microelectronic chip packaging and connection technology. "C4 " stands for Controlled Collapse Chip Connection. C4 is also known as "solder bump" and "flip chip".
The basic idea of C4 is to connect chips, chip packages, or such other units by means of solder balls partially crushed between two surfaces of the units. These tiny balls of electrically conductive solder bridge the gaps between respective pairs of metal pads on the units being connected. Each pad has a corresponding pad on the other unit's surface; the pad arrangements are mirror images. As the units are pressed together the solder balls on the pads of the first unit are pressed against corresponding conductive pads (having no balls) on the second unit, partially collapsing the solder balls and making connections between respective pads.
In C4 the solder balls are formed directly on the metal pads of the one unit. The pads are electrically isolated from other components by the insulating substrate that surrounds each pad. The substrate might be un-doped silicon (Si) or some other material. The bottom of the pad is electrically connected into the chip circuit.
When the two surfaces are aligned and then pushed together, the soft solder bumps are partially crushed. This allows all the connections to be made in one step, in spite of slight variations in the surfaces.
A major application of C4 is in joining semiconductor microchips (integrated circuits) to chip packages. Chips usually are made in rectangular arrays on a mono-crystalline slab of silicon, called a "wafer," which is a thin disc several inches across. Many chips are formed on each wafer, and then the wafer is broken up into individual chips and the chips are "packaged" in units large enough to be handled. The C4 balls are placed on the chips while they are still joined in a wafer.
The wafers are made as large as possible so as to reduce the number wafers that must be processed to make a certain number of chips. For the same reason (among others) the chips are made as small as possible. Thus, the best C4 fabrication system is one that can make thousands of very small, closely-spaced solder balls each precisely placed over a large area.
C4 allows a very high density of electrical interconnections. Unlike earlier techniques which made connections around the perimeter of a chip or a chip package, C4 allows one of more surfaces of a chip or package to be packed with pads. The number of possible connections with C4 is roughly the square of the number that is possible with perimeter connection. Since the C4 balls can be made quite small, about a hundredth of an inch in diameter, the surface density of C4 connections can be on the order of thousands per square inch.
Electrical engineers are constantly placing more and more circuits onto each chip, to improve performance and reduce cost. As the number of circuits on a chip grows, so does the number of connections needed. C4, which allows more connections in a small space than any other technique, is commercially important.
Besides making possible area connection, C4 can also used with perimeter connection techniques such as tape automated bonding (TAB), in which solder balls on a chip are pressed against a pattern of metal foil adhered on a plastic substrate of the chip package. These applications, too, are commercially important.
C4 solder bumps must be mechanically well-fastened to their pads, or they may be torn off when the two surfaces are pushed together. It will be appreciated that a complex device such as a computer may have dozens of chips and hundreds or thousands of C4 solder ball connections, and the entire device may be rendered useless if only one of the balls fails. The attachment of the C4 balls requires careful design.
One method of forming solder bumps uses sputtering or vacuum deposition. Solder metal is evaporated in a vacuum chamber. The metal vapor coats everything in the chamber with a thin film of the evaporated metal. To form solder balls on the substrate, the vapor is allowed pass through holes in a metal mask held over the substrate. The solder vapor passing through the holes condenses onto the cool surface into rounded solder balls. This method requires a high vacuum chamber to hold the substrate, mask, and flash evaporator.
The mask is specially made with high-precision holes, or "vias," for locating the solder balls. The mask will be heated as hot metal vapor released into the vacuum chamber condenses on it. To avoid misalignment of the vias due to thermal expansion, the masks may be made of special metals, and even so the size of the mask is limited. Thus, this method cannot be used for large wafers containing many chips.
An alternative technique for making solder bumps is electrodeposition, also called electrochemical plating or electroplating. This method also uses a mask and forms solder bumps only at the selected sites, but the technique is very different from the evaporation method.
Solder bump electrodeposition requires a first preliminary step, the creation of a continuous "seed layer" of conductive metal adhered onto the insulating substrate. The seed layer is needed to conduct the electricity which deposits solder.
FIG. 1A, labelled "prior art," shows a wafer substrate S whose surface is overlaid with a conductive layer L1 of chromium (Cr). This metal layer, which will function as part of the seed layer for electrodepositing solder balls, might be a ten-thousandth of a millimeter thick. On top of the Cr is deposited a thin "phased" layer L2 of 50% chromium--50% copper (Cr--Cu). Finally, a third layer L3 of pure copper is deposited over all. The Cr, Cr--Cu, and Cu layers are of comparable thicknesses. (FIG. 1A shows atop the seed layers L1-L3 a solder bump B and mask M, which are added in later steps. The first step of coating the substrate S is done on a bare substrate surface.)
The second preliminary step, after the seed layer is laid down, is to form a mask by photolithography. A layer of photoresist is laid onto the seed layer and exposed to light. Un-exposed photoresist can then be washed away to leave the cured photoresist behind as a mask. Cured photoresist is shown in FIG. 1A as part of the mask M.
When the exposed photoresist has been cured and the uncured photoresist has been removed, the mask is complete. The mask has rows of holes where the solder bumps are to be deposited.
The third step is electrodeposition (electroplating) of lead alloy into the mask holes. An electrodeposited solder bump B is shown in FIG. 1A. The solder bump B might be 1/4 millimeter thick; the thickness of the seed layers L1-L3 is exaggerated in FIG. 1A for clarity.
A solder bump B contains a small amount of tin (Sn) and so will adhere well to the uppermost copper layer. The two metals react to form an "intermetallic," Cu.sub.x Sn.sub.y (for example, Cu.sub.3 Sn). The phased Cr--Cu layer L2 holds the Cr and Cu layers together, and the Cr sticks well to the wafer.
After the solder bumps are formed, the mask of cured photoresist is removed. The substrate now is covered with the continuous seed layer and numerous solder bumps.
After the seed layer is deposited over the substrate and the C4 bumps have been formed, the seed layer is desirably removed in between the solder bumps to electrically isolate them. The removal can be done by chemical etching or by electroetching.
FIG. 1B shows the seed layers removed to leave the solder bumps electrically isolated but mechanically fixed to the substrate. This is accomplished by etching the layers L1-L3 away with chemical or electrolytic action; in either case the solder bump B protects the layers under it. FIG. 1C shows a solder ball B formed by melting, or reflowing, the solder bump B of FIGS. 1A-1B. The solder ball is now ready to make contact.
Alloys of titanium (Ti) tungsten (W; also called wolfram) have been used in the prior art as "barrier" layers to protect chip parts during certain processes. Ti--W is metallic and will conduct electricity. A thin film of Ti--W can be applied by conventional microelectronic techniques like sputtering.
If Ti--W is used, it may be desirable to remove the Ti--W at some stage of fabrication. Several inventors have addressed the problem of removing Ti--W.
John Dion, in U.S. Pat. No. 5,130,275, teaches post-fabrication processing of semiconductor chips. His method is intended for solder joining of chips to TAB packages, where the solder is flowed rather than crushed to make the connection. Dion employs a barrier metal layer, of 10% Ti and 90% W by weight, coated over Al or gold (Au) interconnect pads 14 and a passivating layer of SiO.sub.2. A Cu or Au seed layer is coated over the barrier layer. The thickness of metal over the pads is increased by electrodeposition of Cu or Au bumps into holes in a photoresist mask. Solder containing Sn is then deposited on top of the Cu or Au, and then first the seed layer and then the barrier layer are etched away to leave the built-up pads ready for soldering.
The seed layer, if of gold, is removed by chemical etching in 10% potassium cyanide. This etchant attacks the bump as well, but Dion accepts the resulting bump damage as minimal, since the bump is 25 microns thick whereas the seed layer is only 0.3 microns thick.
Dion next etches the Ti--W barrier in an aqueous solution of 30% hydrogen peroxide. (Hydrogen peroxide, or H.sub.2 O.sub.2, is commercially available in 30% concentration.) He notes (column 8, line 37) that peroxide can corrode the solder bead atop the Cu/Au bump. He teaches prevention of corrosion by adjusting the pH of the solution to between 9 and 11 (basic). His preferred solution is 7% oxidized ammonium persulfate and 1% to 2% hydrogen peroxide, with the pH adjusted to between 9 and 11 by adding ammonium hydroxide. However, Dion's etchants will attack aluminum.
A 10% Ti--90% W barrier layer is also taught by James Watson in U.S. Pat. No. 5,041,191. Watson uses the Ti--W layer to prevent undesirable intermetallics that form when Au, Cu or Al contacts are deposited directly onto a thin-film resistor of nickel-chromium alloy. Watson's Ti--W etchant is 5 g of cupric sulfate (CuSO.sub.4), 10 ml ammonium hydroxide (NH.sub.4 OH), 100 ml glycerol, and 125 ml deionized water.
This solution does not affect the nickel chromium, according to Watson. Like Dion's however, Watson's solution is alkaline and will attack aluminum.
Stephen Pyke, in U.S. Pat. No. 4,671,852, teaches an etchant composed of hydrogen peroxide, EDTA, and ammonium hydroxide for removing a thin (0.05-0.10 microns) film of 10%-30% by weight Ti and 90%-60% by weight W. Pyke's device is a chemically-sensitive SGFET structure with an internal cavity. Etchant is introduced into the cavity, which also contains a noble metal (platinum) and an aluminum oxide or silicon oxide "fugitive" layer. The etchant is intended to selectively etch the Ti--W film.
Pyke's etchant is 0.1 molar EDTA, 30% hydrogen peroxide, and concentrated ammonium hydroxide mixed in a respective volume ratio of 10:3:2. Pyke states that the pH should be less than 11 (not too basic). Pyke also teaches the use of other complexing agents besides EDTA, such as carboxylates, bipyridines, etc., but he gives no formulas or other details.
Pyke's invention, too, will attack aluminum.
U.S. Pat. No. 4,814,293, issued to Jacques Van Oekel, also teaches chemical etching of 10% Ti--90% W. He notes that hydrogen peroxide causes inhomogeneous etching, and in particular the undercutting or underetching, when Ti--W films are layered between other metals, is irregular. The agitation commonly used is ineffective in reducing the uneven results, and he advocates stagnant liquid etchants. Van Oekel buffers the peroxide solution to a pH value between 1 and 6 (acidic). His preferred buffering compounds are acetic acid and ammonium acetate. He also uses citric acid and sodium hydroxide. The etch rate is varied with the pH.
Van Oekel's solution will severely attack lead-tin (Pb--Sn) alloys such as solder, and this patent does not address selective etching of Ti--W in the presence of Pb--Sn.
Minford et al., in U.S. Pat. No. 4,554,050 teach the use of Ti etchants in fabricating waveguides. Their etchant is composed of EDTA, water, hydrogen peroxide, and ammonium hydroxide. One formula they present is 2.5 g of disodium EDTA in 100 ml of deionized water (a 0.067M solution) with 10 g hydrogen peroxide and 4.2 g ammonium hydroxide. The pH is about 10.
The etch rate is controlled by varying the OH concentration and the temperature. Minford et al. tested their solution at 20 degrees C. (room temperature) and at 60 degrees C.
Minford et al. states that their solution will etch aluminum. Moreover, their solution will attack Pb--Sn as well as aluminum.